An organic light emitting diode (OLED) typically include a cathode and an anode, and light emitting units between the cathode and the anode. An OLED can be a single unit OLED or a tandem OLED. A single unit OLED has only one light emitting unit, which includes a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer. In an OLED, an electric field is applied between the anode and the cathode to inject electrons from the cathode into the light emitting layer and holes from the anode into the light-emitting layer. The electrons and the holes then recombine together in the light emitting layer to generate excitons. When the excitons return to the ground state, their energy is released in the form of light.